update_git.bash

#!/usr/bin/env bash

# Returns true if git needs to be updated; false otherwise
function update_git() {
	pushd "$1"
	
	git remote update

	UPSTREAM=${1:-'@{u}'}
	LOCAL=$(git rev-parse @)
	REMOTE=$(git rev-parse "$UPSTREAM")
	BASE=$(git merge-base @ "$UPSTREAM")

	if [ "$LOCAL" = "$REMOTE" ]; then
	    echo "Up-to-date"
		res=0
	elif [ "$LOCAL" = "$BASE" ]; then
	    echo "Need to pull"
		res=1
	elif [ "$REMOTE" = "$BASE" ]; then
	    git reset --hard HEAD
		git clean -f -d
		res=1
	else
	    git reset --hard HEAD
		git clean -f -d
		res=1
	fi
	
	popd

	return "$res"
}

Based on: https://stackoverflow.com/a/3278427